Tempo and mode of evolutionary divergence in modern and Holocene Vancouver Island marmots (Marmota vancouverensis) (Mammalia, Rodentia)

Marmota vancouverensis is the only insular species among the 14 species of marmots. The evolutionary history of this species is unresolved. Although M. vancouverensis is strongly differentiated in osteological and other morphological characters, its low genetic divergence suggests recent evolution from an ancestral continental species. We used geometric morphometric techniques to assess the morphology of hemimandibles from 239 modern M. vancouverensis, Marmota caligata, Marmota flaviventris, Marmota olympus and 30 Holocene (9435-735 cal. yr bp) subfossil M. vancouverensis. Our results confirm that the mandible of M. vancouverensis is strongly differentiated in shape from continental marmot species, but similar in size to its mainland sister species M. caligata. Temporal variation in size and shape over the past 2500 years among allochronic samples of M. vancouverensis was minimal suggesting that the morphological divergence of this species occurred in a period of rapid change following its isolation from mainland populations in the late Pleistocene. Selection pressures associated with environmental changes and founder effects and genetic drift resulting from population bottlenecks created by population declines and habitat fragmentation are hypothesized as factors contributing to the morphological divergence of this species.